In automatic transmissions the shifting of the various gear ratio steps generally takes place by hydraulic pressurization of shift elements such as clutches or brakes. The shift elements, together with an oil supply system, form a hydraulic system. To change the gear ratio, certain shift elements in the hydraulic system are relieved from hydraulic pressure and other shift elements are filled with an operating medium and acted upon by a hydraulic pressure. Since the gear ratio change has to be completed within a finite, limited time, the filling of the shift elements must take place correspondingly quickly, and this entails producing a high volume flow in the hydraulic system. The volume flow is produced by a pump of the oil supply system, as is the hydraulic pressure for pressurizing the shift elements. For this purpose as a rule only one pump is provided in an automatic transmission, in most cases in the form of a displacement pump with a fixed displacement volume. The displacement pump is usually a gearwheel pump, preferably an internal gearwheel pump because of its more compact structure. The pump rotates in proportion to the input speed of the transmission, so the volume flow delivered by the pump increases linearly with the input speed. The function of the volume flow with input speed, is also called the delivery characteristic in what follows. Conventionally, the displacement volume of the pump is chosen such that the oil demand of the hydraulic system can be covered at all rotation speeds and to ensure all operating functions. Such operating functions are for example the engagement of a gear ratio step or a shift process for changing gear ratios, with the brief demand for a high volume flow described. This peak demand must be covered by the delivery characteristic of the pump, and consequently, other than during the operating functions, the volume flow delivered exceeds the needs of the automatic transmission and thus, in combination with the pressure produced, constitutes an energy loss with corresponding adverse impacts on the efficiency of the hydraulic system and the transmission.
In one possible solution of this problem, the displacement volume of the pump is made so small that only the demand during steady-state operation of the automatic transmission, i.e. not during gear ratio changes, is covered. With this comparatively small volume flow the automatic transmission is lubricated and cooled, and any leakage losses in the hydraulic system are made up. The momentary peak demand is covered for a limited time by an auxiliary pump. The auxiliary pump is preferably driven by an electric motor, since this can be switched on or off according to need. In a conventional automatic transmission the motor is an auxiliary electric motor. If the automatic transmission forms part of a hybrid drivetrain in which an electric machine acting as a motor is also arranged, then the auxiliary pump can be driven by the machine.
To keep down the construction effort and costs, the auxiliary electric pump is not designed for the maximum peak pressures that occur in the hydraulic system. This can relate to both the design of the auxiliary pump and the size of the electric motor. To avoid overloading due to a system pressure that is too high, according to the prior art a pressure line of the electric auxiliary pump is connected via a pressure-relief valve to its suction line or to the transmission sump. The pressure-relief valve is set to open as soon as the system pressure exceeds a still acceptable pressure limit value. In this way the pressure acting on the electrical auxiliary pump is limited and overload thereof is avoided.
The disadvantages of this solution are the structural space required in the transmission for fitting the pressure-relief valve in the hydraulic system, the additional components and the costs for the pressure-relief valve.